1. Field of the Invention
The present invention relates to a temperature detector to detect a temperature of a sensor element in such a manner that an internal resistance value of an exhaust gas sensor to detect a concentration of oxygen contained in exhaust gas discharged from an internal combustion engine is measured and the thus measured internal resistance is converted into the temperature of the sensor element.
2. Description of the Related Art
A technique is well known in which a concentration of oxygen contained in exhaust gas discharged from an internal combustion engine is detected, and the exhaust gas is purified and the rate of fuel consumption of the internal combustion engine is improved by conducting feedback control with an air fuel ratio of mixture supplied to the internal combustion engine according to the thus detected concentration of oxygen. This technique is widely applied to internal combustion engines for automobile use. Concerning the exhaust gas sensor to detect the concentration of oxygen, it is necessary to keep the temperature in an activating region so that the detection characteristic of the oxygen concentration can be stabilized. Therefore, a heater is incorporated into the exhaust gas sensor and an electrical current is supplied to the heater being controlled so as to keep the temperature of the exhaust gas sensor at a predetermined value. In order to control an intensity of this electrical current, it is necessary to measure the temperature of the sensor element. With respect to this temperature measuring technique, various methods have been disclosed until now.
For example, according to Japanese Unexamined Patent Publication No. 9-292364, as the method of measuring the internal resistance value by which the internal resistance value of the exhaust gas sensor is detected so as to calculate the temperature of the sensor element from the thus measured internal resistance value, there is disclosed a technique in which a voltage impressed upon the exhaust gas sensor in the case of detecting the concentration of oxygen is changed over to a voltage used for detecting a resistance value by a predetermined time constant, and the internal resistance value of the exhaust gas sensor is detected from a state of a change in the voltage and current. According to Japanese Unexamined Patent Publication No. 7-119736, there is disclosed a technique in which a nonlinear type exhaust gas sensor, which is called a xcex type exhaust gas sensor, is used and the temperature of the exhaust gas sensor is indirectly estimated by using parameters of the running state of an internal combustion engine such as a quantity of sucked air and a rotating speed.
Further, Japanese Unexamined Patent Publication No. 8-313476 discloses a technique by which the temperature of a sensor element is indirectly estimated in such a manner that a heater resistance is detected from a voltage and current of a heater arranged close to the exhaust gas sensor so that a heater temperature is calculated. Further, Japanese Unexamined Patent Publication No. 1-172746 discloses a technique by which the temperature of an exhaust gas sensor, which is arranged close to a heater, is indirectly estimated in such a manner that a target resistance value of the heater according to a flow rate of exhaust gas, that is, a target of a temperature of the heater is set and an electrical current is controlled so that the heater resistance value can be the target value.
In the prior art described above in which the temperature of an exhaust gas sensor is indirectly measured from the resistance value of a heater, a correlation between the heater temperature and the exhaust gas sensor is changed by the flow rate and the temperature of exhaust gas. Therefore, even if the target resistance value of the heater is set according to the flow rate of exhaust gas as disclosed in Japanese Unexamined Patent Publication No. 1-172746, it is impossible to detect an accurate temperature of the exhaust gas sensor. In the method described in Japanese Unexamined Patent Publication No. 9-29236 in which a temperature is calculated by measuring an internal resistance value of the exhaust gas sensor itself, the internal resistance value and the correlation between the internal resistance value and the temperature are greatly changed by the fluctuation and the change with age. Therefore, it is impossible to detect an accurate temperature only by measuring the internal resistance value and converting it into temperature.
The present invention has been accomplished to solve the above problems. It is an object of the present invention to provide a temperature detector for an exhaust gas sensor by which a temperature can be accurately detected by conducting measurement while the fluctuation of the internal resistance value is being corrected and also the change with age is being corrected, and further a warning against deterioration is outputted when the internal resistance value is greatly changed with age.
A temperature detector for an exhaust gas sensor comprises: an exhaust gas sensor for detecting a concentration of oxygen contained in exhaust gas discharged from an internal combustion engine; a temperature sensor for measuring an environmental temperature which is substantially the same as the exhaust gas sensor; a resistance measuring means for measuring an internal resistance value of the exhaust gas sensor; a timer means for measuring an operation stop time of the internal combustion engine; and a control means for detecting the internal resistance value of the exhaust gas sensor and the environmental temperature at each operation starting point when a period of operation stop conducted by the timer means is longer than a predetermined period, for calculating a characteristic of the internal resistance value versus the temperature of the exhaust gas sensor from both the internal resistance of the exhaust gas sensor and the environmental temperature, for renewing and storing them, and for calculating a temperature of the exhaust gas sensor from the internal resistance of the exhaust gas sensor in operation, which is measured in operation, and also from the characteristic of the internal resistance value versus the temperature.
A temperature detector for an exhaust gas sensor comprises: an exhaust gas sensor for detecting a concentration of oxygen contained in exhaust gas discharged from an internal combustion engine; a temperature sensor for measuring an environmental temperature which is substantially the same as the exhaust gas sensor; a resistance measuring means for measuring an internal resistance value of the exhaust gas sensor; a timer means for measuring an operation stop time of the internal combustion engine; and a control means for detecting the internal resistance value of the exhaust gas sensor and the environmental temperature at each operation starting point when a period of operation stop conducted by the timer means is longer than a predetermined period, for calculating a characteristic of the internal resistance value versus the temperature of the exhaust gas sensor from both the internal resistance of the exhaust gas sensor and the environmental temperature, for renewing and storing them, and for calculating a temperature of the exhaust gas sensor from the internal resistance of the exhaust gas sensor in operation, which is measured in operation, and also from the characteristic of the internal resistance value versus the temperature, wherein the control means has a discriminating function to discriminate an operation state between a test mode and an operation mode, the control means stores an internal resistance value of the exhaust gas sensor, which has been measured in the first motion of the operation mode after the completion of the test mode, as a reference value, and the deterioration of the exhaust gas sensor is judged when this reference value is compared with the internal resistance value of the exhaust gas sensor measured at each operation staring point.
In a temperature detector for an exhaust gas sensor, the temperature sensor is an outside air temperature sensor mounted on a vehicle or a cooling water temperature sensor.
In a temperature detector for an exhaust gas sensor, the timer means for measuring an operation stop period utilizes a signal obtained from a digital clock mounted on a vehicle.
In a temperature detector for an exhaust gas, the timer means for measuring an operation stop period utilizes a heat-radiating time in which heat is radiated so that a heating section of the temperature detector can be cooled to a predetermined temperature after the stop of operation.